The present invention generally relates to mobile communications technology, and more particularly to a cell initial search method of user equipment (UE) in a CDMA digital cellular mobile communications system with a training sequence (pilot).
In a digital cellular mobile communication system, after a UE is turned on, a cell initial search is made first. The purposes of the cell initial search are to select a suitable working frequency and to obtain downlink synchronization between the UE and a base station at this working frequency. Only in this way can the UE correctly receive a message sent by the base station.
Additionally, in a real digital cellular mobile communication system, the main clock in the UE and the main clock in the base station are completely independent. Even if both are working at the same working frequency, there is a carrier deviation (also called frequency difference or frequency deviation) between them. During demodulation, if the UE cannot implement relatively accurate carrier deviation recovery (or said calibration, correction), then there will be a carrier component remaining in the baseband signal, this will affect the processing of the baseband signal, further causing code error and make the UE unable to correctly receive information sent by a base station.
Therefore, for a digital cellular mobile communication system, a UE must perform the following work during the cell initial search: lock-in a working frequency, obtain downlink synchronization with the base station at the locked working frequency, and correct carrier frequency deviation.
Naturally, during a real cell initial search, because the independent main clocks of a base station and UE drift along with time, carrier frequency deviation also must also be traced at the same time.
In a real CDMA cellular mobile communication system, a pilot channel generally performs the downlink synchronization. The conventional implementation of a downlink synchronization is: first, locking-in at a working frequency, solving the correlation between the whole data frame received and a preset pilot sequence (training sequence), and then continuously sliding the working frequency for solving the correlation until the correlation peak is greater than a preset threshold. The downlink synchronization is then performed at this working frequency. The working frequency where the correlation peak is located is the UE receiving position.
In any CDMA cellular mobile communication system, a correlation operation is needed for synchronization. Nevertheless, the conventional correlation operation has the following limitation: as the correlation operation is slid at each chip or even fractional chip level of the whole data frame, the operation volume is huge and a long calculation time is required. Besides, as the correlation operation is taken for the whole data frame, it increases the probability of an error decision, especially, in the Time Division Duplex CDMA (TDD-CMDA) system. In a TDD-CDMA system, suppose, for example, near UE xe2x80x9cAxe2x80x9d there is another UE xe2x80x9cBxe2x80x9d in conversation. As the distance between UE xe2x80x9cAxe2x80x9d and UE xe2x80x9cBxe2x80x9d is short, UE xe2x80x9cAxe2x80x9d receives a more powerful signal from UE xe2x80x9cBxe2x80x9d than the signal power sent by the base station. This leads to a mistaken decision about the correlation peak position that is not the real receiving position of the UE, and produces mistaken downlink synchronization information.
In general, carrier frequency deviation is corrected at a digital demodulator (for general situations, a certain degree of carrier deviation will not affect downlink synchronization, but will affect the demodulated information). An analog phase-locked circuit, which is mature technology and well known in the art, is conventionally used. Disadvantages of this solution are: it is difficult to take account of performance and capture bandwidth at the same time, it is sensitive to carrier jitter and it is a complex hardware circuit.
In the China Patent CN 97115151.2 named xe2x80x9cA Method And Device For Carrier Recovery And Compensation In A Frequency Spread Communication System,xe2x80x9d a digital correction method for carrier frequency deviation has been proposed. Nevertheless, the method makes an optimal estimation under a channel model without noise and multipath interference, and is not suitable for a cellular mobile communication system.
The purpose of the invention is to provide a cell initial search method for a CDMA digital mobile communication system. The method improves the conventional cell initial search method, i.e. proposes a solution for downlink synchronization and carrier deviation correction during cell initial search. With this solution the UE can rapidly and accurately perform downlink synchronization with a base station and has a better effect for carrier deviation correction.
An implementation of the invention can be as follows:
A cell initial search method for a digital mobile communication system with CDMA is used for user equipment so that such user equipment may correctly receive information sent by a base station. The method is characterized in that the user equipment selects a working frequency, and obtains downlink synchronization with the base station at the working frequency, comprising:
a. based on a power characteristic window value method of training sequence, a range of downlink training sequence timeslot is decided first;
b. in the range, by calculating correlation of received data and training sequence, an accurate receiving position of the user equipment is obtained.
The said xe2x80x9cbased on a power characteristic window value method of training sequence,xe2x80x9d comprises:
a. in a base station frame structure, increasing the transmitting power of synchronization symbols in the downlink pilot sequence timeslot (DwPTS), and making no transmitting power on protected symbols located before and after the synchronization symbols in the DwPTS;
b. when receiving, user equipment first searches the power characteristic window values of the DwPTS; after a position range of the synchronization symbols has been discovered the correlation operation is only made near the position.
The said xe2x80x9csearching the power characteristic window values of the DwPTS to discover a position range of the synchronization symbols,xe2x80x9d includes:
UE locking-in at a working frequency, then receiving a data frame; calculating each synchronization symbols"" power in the DwPTS; calculating power characteristic window values at each synchronization symbols"" position; calculating an average power characteristic window value for the whole data frame; searching for a minimum value of the power characteristic window values at all synchronization symbols position of the whole data frame received; deciding whether the ratio of average power characteristic window value and minimum power characteristic window value is greater than a threshold value, and, if it is, then the position of the minimum power characteristic window value is the beginning position of the DwPTS; solving correlation near the beginning position to obtain an accurate receiving beginning point and end the downlink synchronization.
The said xe2x80x9ccalculate each synchronization symbol power,xe2x80x9d is to assume that the receiving moment is a synchronization symbol beginning point, then adding the powers of all chips belonging to the symbol to obtain the power of each synchronization symbol.
The said xe2x80x9ccalculate power characteristic window value at each synchronization symbol position,xe2x80x9d is for the whole data frame received, calculating each position power characteristic window value R(i) with symbol level sliding at each position with the following formula:       R    i    =            (                                    ∑                          k              =              i                                      i              +              N              -              1                                ⁢                      P            ⁢                          (              k              )                                      +                              ∑                          k              =                              i                +                N                +                M                                                    i              +                              2                ⁢                N                            +              M              -              1                                ⁢                      P            ⁢                          (              k              )                                          )        /                  ∑                  k          =                      i            +            N                                    i          +          N          +          M          -          1                    ⁢              P        ⁢                  (          k          )                    
wherein i represents a real receiving position, P(k) represents power value of each symbol, N and M are characteristic window parameters.
The said xe2x80x9ccalculate power characteristic window value at each synchronization symbol position,xe2x80x9d is, calculating the power characteristic window value at each position based on each chip power with chip level sliding.
Another implementation of the invention can be as follow:
A cell initial search method for a digital mobile communication system with CDMA is used for a user equipment so that such user equipment may correctly receive information sent by a base station. The method is characterized in that the user equipment traces carrier deviation between the user equipment and the base station, and corrects the carrier deviation in a digital demodulator, comprising:
a. the carrier deviation is estimated by software, and hardware is adjusted by a decision and feedback method;
b. the multipath and multiple access interference is suppressed based on a joint detection method for carrier deviation correction, and the carrier deviation is corrected to a range required by baseband demodulation.
The said Step a includes: estimating carrier deviation of each data frame by software, calculating an adjustment value for the hardware, and adjusting automatic frequency control hardware in the digital demodulator with the calculated adjustment value.
The following formula is used to estimate frequency differences of each data frame by software:       A    ⁢          xe2x80x83        ⁢          ⅇ                        -          j                ⁢                  xe2x80x83                ⁢        k        ⁢                  xe2x80x83                ⁢        α              ⁢          1      L        ⁢                  ∑        L            ⁢                                                  [                                                I                  ⁢                                      (                    i                    )                                                  +                                  j                  ⁢                                      xe2x80x83                                    ⁢                                      Q                    ⁢                                          (                      i                      )                                                                                  ]                        *                    ⁡                      [                                          I                ⁡                                  (                                      i                    +                    K                                    )                                            +                              j                ⁢                                  xe2x80x83                                ⁢                                  Q                  ⁢                                      (                                          i                      +                      K                                        )                                                                        ]                          *              ,
wherein xcex1 represents an estimated frequency difference, I and Q are orthogonal demodulation signals, and L is a statistic length.
The following formula is used to calculate a hardware adjustment value, i.e. for calculating adjustment value of a carrier frequency difference:
fa(n)=fe(n)*coefk(n),
wherein fe(n) is the estimated frequency difference for the nth data frame received, the range of adjustment coefficient coefk is between 0xcx9c1, and when k is greater, select a smaller coefk.
The said xe2x80x9cmultipath and multiple access interference is suppressed based on joint detection method for carrier deviation correction, and the carrier deviation is corrected to a range required by baseband demodulation,xe2x80x9d includes: a training sequence midamble of data burst inserted into each frame; the user equipment suppresses multipath and multiple access interference with joint detection technology, and symbols near the training sequence midamble are demodulated; with carrier frequency difference information included in these symbols, automatic frequency control hardware is adjusted.
The said xe2x80x9csuppress multipath and multiple access interference with joint detection technology, and symbols near the training sequence midamble is demodulated,xe2x80x9d further includes: demodulating data with the joint detection technology and obtaining P characters before and after the training sequence midamble, and recorded as X(1) . . . X(P) and Y(1) . . . Y(P), respectively; calculating Xi(n)=X(n)/Xj(n), Yi(n)=Y(n)/Yj(n), wherein Xj(n)=Yj(n)=xc2x1xcfx80/4, xc2x13xcfx80/4; obtaining carrier frequency difference direction by the formula:       Z    =                  ∑                  n          =          1                P            ⁢              Y        ⁢                  xe2x80x83                ⁢                              i            ⁢                          (              n              )                                /          X                ⁢                  xe2x80x83                ⁢                  i          ⁢                      (            n            )                                ;
setting an adjustment step length based on the calculated carrier frequency difference direction; adjusting automatic frequency control hardware with the adjustment step length and the obtained frequency difference direction.
Another implementation of the invention also can be as follows:
A cell initial search method for a digital mobile communication system with CDMA comprises: a user equipment selects a working frequency point and obtains downlink synchronization with a base station at the working frequency; the user equipment traces carrier deviation between the user equipment and the base station, and corrects the carrier deviation in a digital demodulator.
Wherein the said xe2x80x9cobtains downlink synchronization with a base station,xe2x80x9d comprises:
a. based on a power characteristic window value method of training sequence, a range of downlink training sequence timeslot is decided first;
b. in the range by calculating correlation of received data and training sequence, an accurate receiving position of the user equipment is obtained.
The said xe2x80x9ccorrect the carrier deviation between the user equipment and the base station in a digital demodulator,xe2x80x9d comprises:
a. the carrier deviation is estimated by software, and hardware is adjusted by a decision and feedback method;
b. the multipath and multiple access interference is suppressed based on joint detection method for carrier deviation correction, and the carrier deviation is corrected to a range required by baseband demodulation.
The said xe2x80x9cbased on a power characteristic window value method of training sequence,xe2x80x9d comprises:
a. in the base station frame structure, increasing transmitting power of synchronization symbols in the downlink pilot timeslot (DwPTS), and making no transmitting power on protected symbols located before and after the synchronization symbols in the DwPTS;
b. when receiving, user equipment first searches the power characteristic window values of the DwPTS, and after the position range of the synchronization symbols has been discovered, the correlation operation is made only near the position.
The invention is a cell initial search method for a CDMA mobile communication system, and is a downlink synchronization method for a CDMA mobile communication system. The method implements locking a working frequency point during a cell initial search, obtaining downlink synchronization with the base station and recovering carrier frequency difference between the base station and user equipment. The method of locking a working frequency point and obtaining downlink synchronization with the base station is: first, deciding a rough range of the training sequence by using a power characteristic window value method based on the training sequence, then solving correlation of a data frame received and the training sequence to obtain the accurate receiving position and to complete a downlink synchronization with the base station. The method of recovering carrier frequency difference between the base station and user equipment is a carrier frequency difference correction method based on joint detection technology. Implementing several steps of the two methods above (or any one of them) will implement a cell initial search of the invention and user equipment completes downlink synchronization rapidly and accurately.
The method of the invention is a cell initial search method mainly directed toward a mobile communication system with training sequence.